In the last decade luminescent microporous Si gained considerable interest and has been applied, for example, in solar cell applications or in optical devices.
Nowadays the interest in porous semiconductor materials is fuelled by the discovery of other kinds of pores and their corresponding properties.
Products relying on porous Si are readily commercially available. Meanwhile also other semiconductor materials are receiving more interest, such as III-V compounds or SiGe alloys. In porous form these compounds or alloys would also exhibit properties with a large potential for applications.
R. B Wehrspohn et al disclose in “Electrochemically Prepared Pore Arrays for Photonic-Crystal Applications”, (Material Research Bulletin, August 2001, p 623–626) the use of macropores to form so-called photonic crystals. These macropores are formed by electrochemical etching. European patent application EP 1 132 952 discloses the formation and lift-off of porous silicon layers. These layers are used to manufacture Silicon-On-Insulator (SOI) substrates or fabricate photovoltaic cells on low-cost substrates. First a double-porous top layer is formed on a seed wafer by electrochemical etching. This double-porous layer can optionally be used as a starting layer to form epitaxial layers there upon. After bonding the seed wafer to a handle wafer the double-porous layer is used as a cleavage layer to split off the non-porous part of the seed wafer.
As already indicated above, one technique to produce porous materials is electrochemical etching, also known as anodization. It is considered to be the most appropriate and versatile one.
M. Christopher et al. discuss in “Crystal Orientation Dependence and Anisotropic Properties of Macropore Formation of p- and n-Type Silicon”, Journal of Electrochemical Society 149 (2001) E267–E275, the formation of macropores in p- or n-type silicon substrates. The method requires at least one surface of this conductive substrate to be put into contact with an aqueous or organic HF-containing electrolyte, while a voltage difference is applied over the substrate and the electrolyte. By selecting the composition and the pH of the anodic etching solution and dependent on the electric biasing conditions, the diameter and the pitch of thus formed macropores can be controlled.
However these pores will preferentially have a <100>-crystal orientation and are thus crystal orientation dependent.
Furthermore electrochemical etching yields macropores with a large diameter, typically above 1 micrometer, which is sometimes to be avoided. Furthermore the etch rate of the electrochemical etching depends on the substrate dopant type and concentration.
Another technique to produce porous materials is disclosed by T. Sato, et al., in “SON (silicon on nothing) MOSFET using ESS (Empty Space in Silicon) technique for SOC applications (IEDM 2001, p 809–812). This paper illustrates the use of microchannels to form, depending on the pattern of these microchannels or macropores, spherical, pipe-shaped or plate-shaped empty spaces in the bulk of a semiconductor substrate. On the thin surface layer of semiconductor material overlying these empty spaces a transistor can be made, which is electrically insulated from the underlying semiconductor substrate by these subsurface voids. During processing first an oxide layer is formed over the semiconductor substrate. Openings are then defined in this oxide layer using lithographic processing and Reactive Ion Etching (RIE). Through these openings in the oxide trenches with a high aspect ratio are etched in the semiconductor substrate. Depending on the pattern of the thus formed trenches or microchannels empty spaces of various shapes can be obtained.
Although the use of lithographic processing allows forming microchannels with a small diameter, the manufacturing cost will however increase with shrinking diameter of the trenches.
For diameters of 150 nm or less, one have to use DUV lithography with requires dedicated masks sets, photosensitive resists and lithographic tools, all of which are very expensive.
An aim of the present invention is therefore to offer a method to form macropores having a small diameter, preferably less than 400 nanometers, without the need for lithographic processing.
One additional advantage of this method is that these macropores have a predetermined diameter, and that this method is independent of the substrate material.